The present invention relates generally to an apparatus for a hydrogen fueled vehicle. Even more particularly, the present invention relates to a hydrogen fueled vehicle that employs expansion and contraction of a volume of coolant within a bipolar plate assembly to pump coolant throughout a cooling system in the event of a coolant pump failure.
Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation of hydrogen with oxygen in air. Electric power is provided to an electric motor for vehicle propulsion. The only byproducts produced by such a system are pure water and off-heat. The off heat is generally rejected to the environment by virtue of a liquid coolant loop and a typical automotive radiator. Hydrogen is a very attractive fuel because it is clean and it can be used to produce electricity efficiently in a fuel cell. The automotive industry has expended significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Vehicles powered by hydrogen fuel cells would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied as a reactant through a flow path to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied as a reactant through a separate flow path to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal such as platinum, are placed at the anode and cathode to facilitate the electrochemical conversion of the reactants into electrons and positively charged ions (for the hydrogen) and negatively charged ions (for the oxygen). In one well-known fuel cell form, the anode and cathode may be made from a layer of electrically-conductive gaseous diffusion media (GDM) material onto which the catalysts are deposited to form a catalyst coated diffusion media (CCDM). An electrolyte layer separates the anode from the cathode to allow the selective passage of ions to pass from the anode to the cathode while simultaneously prohibiting the passage of the generated electrons, which instead are forced to flow through an external electrically-conductive circuit (such as a load) to perform useful work before recombining with the charged ions at the cathode. The combination of the positively and negatively charged ions at the cathode results in the production of non-polluting water as a byproduct of the reaction. In another well-known fuel cell form, the anode and cathode may be formed directly on the electrolyte layer to form a layered structure known as a membrane electrode assembly (MEA).
One type of fuel cell, called the proton exchange membrane (PEM) fuel cell, has shown particular promise for vehicular and related mobile applications. The electrolyte layer of a PEM fuel cell is in the form of a solid proton-transmissive membrane (such as a perfluorosulfonic acid membrane, a commercial example of which is Nafion™). Regardless of whether either of the above MEA-based approach or CCDM-based approach is employed, the presence of an anode separated from a cathode by an electrolyte layer forms a single PEM fuel cell; many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output.
In a fuel cell stack, a plurality of fuel cells are stacked together in electrical series while being separated by a gas impermeable, electrically conductive bipolar plate. In some instances, the bipolar plate is an assembly formed by securing a pair of thin metal sheets having reactant flow paths formed on their external face surfaces. Typically, an internal coolant flow path is provided between the metal plates of the bipolar plate assembly. Various examples of a bipolar plate assembly of the type used in PEM fuel cells are shown and described in commonly-owned U.S. Pat. No. 5,776,624, the contents of which are hereby incorporated by reference.
Typically, the cooling system associated with a fuel cell stack includes a circulation pump for circulating a liquid coolant through the fuel cell stack to a heat exchanger where the waste thermal energy (i.e., heat) is transferred to the environment. The thermal properties of typical liquid coolants require that a relatively large volume be circulated through the system to reject sufficient waste energy in order to maintain the temperature of the stack within an acceptable range, particularly under maximum power conditions. However, failure of a coolant pump presents significant operating challenges. Currently, when a coolant pump failure is detected, a fuel cell system is generally operated under reduced power conditions to limit heat production until a maximum temperature threshold is exceeded. Once the system temperature has reached maximum allowable levels, the system is shut down. System shut-down may strand the vehicle operator, leading to a walk-home event.
A practical challenge for fuel cell-powered vehicles is maintaining vehicle motive power in the event of a coolant pump failure. A need exists for a fuel cell system that allows for continuous operation of the fuel cell-powered vehicle in the event of a coolant pump failure.